Thermal and in-reactor densification of UO2: mechanisms and experimental results

The modelling of fuel behaviour and the tailoring of a pellet microstructure for an optimum balance between densification and swelling requires an accurate description of both processes. A detailed densification model is presented which describes the dependence of densification on time, burn-up, temperature, density, pore size distribution, and grain size. Densification is in principle a two-step mechanism: (1) generation of an excess vacancy concentration around the pores, and (2) partial migration of these vacancies to the grain boundaries. Excess vacancy generation is thermally activated as well as fission induced, but at medium and low temperatures the fission induced generation exceeds the thermal generation. The vacancy migration is purely thermally activated at very low temperatures, where an athermal irradiation component of migration exists. The fraction of fine porosity strongly controls the initial densification rate. Small pores will shrink more rapidly than large pores; the latter ones may even increase in size. Thus, a characteristic shift of the pore size distribution results. Experimental verification is obtained from thermal resinter tests, fuel performance tests, and isothermal irradiation tests. The model also sheds more light upon the correlation between thermal resinter testing and in-reactor densification. (Auth.)